Patent GB-856973-A (GB)
Title: Improvements relating to high frequency electron discharge tubes
IPC: H01J23/02
Filed: 1957-02-22
Abstract: 856,973. Velocity modulated tubes. VARIAN ASSOCIATES. Feb. 22, 1957 [Feb. 24, 1956], No. 6106/57. Class 39(1). [Also in Group XL(c)] A klystron or travelling wave tube has a gun comprising a dished cathode 3, Fig. 1, an accelerating electrode 9 and a centrally apertured gating electrode 7 encircling the beam path but presenting no obstruction thereto and disposed between the cathode 3 and accelerating electrode 9, and a source of control pulses is connected between the cathode -and gating electrode to turn the beam on and off; to turn the beam on the gating electrode 7 is pulsed to the same potential as the cathode 3. A second gating electrode in the form of a ring or post also offering no obstruction to the beam, and at the same or different potential as the first gating electrode 7 may be centrally disposed within the first gating electrode 7 which also serves as a focusing electrode, Fig. 3 (not shown), 4 is a focusing electrode. The device is used as a pulsed amplifier or pulse position modulator in radar or pulse communication systems. To achieve short rise and fall times for the leading and trailing edge of the pulses the resistance of the circuit charged by the condenser Co (representing the inter-electrode capacitances and capacities associated with the leads of modulator triode 22, Figs. 4, 6) which may be equal approximately to the resistance of the klystron amplifier tube, Fig. 4, or to about half this value, Fig. 6, is made as small as possible. The invention is particularly applicable to high power klystrons or travelling wave tubes with high density electron beams. In Fig. 4 both tubes 21, 22 are initially cut off and 22 is turned on by a positive pulse applied to its gating electrode 7 ; this causes the potential of the klystron cathode to fall from -6À5 KV to -10 KV i.e. approximtaely the potential of the klystron gating electrode 7 (-10 KV) so that the klystron amplifier is unblocked, while at the end of the pulse to 25 tube 22 is blocked and the klystron cathode potential returns to -6À5 KV so blocking it. The beam cut-off in the klystron is in fact exponential as the potential of its cathode rises from -10 KV to -6À5 KV exponentially. The equivalent resistance determining the pulse fall time is that of R 21  in parallel with R 1  and R 2  which is approximately equal to R 21 . In a modification, Fig. 6, a third tube 32 namely a restorer tetrode (screen grid connected to anode so functions as triode) is added to the circuit and a reference resistance 31 of 70 ohms is placed between the anode of 22 and the cathode of 26. When tube 22 (and consequently tube 21) is turned on by a pulse applied to its control grid 25 the restorer tube grid goes negative and biases it to cut off. When the pulse ends, restorer tube 32 is unblocked and restores the cathode 26 of the klystron to its original potential of -6À5 KV so blocking the klystron again. In this case the equivalent resistance determining the fall time is the equivalent resistance of klystron 21 and of restorer tube 22 in parallel therewith, which for R 21  = R 22  is approximately ¢ R 21  so that the fall time is halved. In one gun construction, Fig. 1, the indirectly heated cathode 3 has annular focusing electrode 4 at cathode potential or netative thereto and the control electrode 7 spaced from 4 by 0À015" and carried on a cylinder 6 of alumina ceramic. The facing ends of 4, 7 are rounded and polished to minimise flash-over. The klystron has input resonator 12 fed by waveguide 13, a plurality of intermediate resonators which are excited by the beam and react back on the beam to cause further velocity modulation, and an output resonator. The gun anode 11 also serves as a pole piece of a permanent focusing magnet (not shown). In another gun, Fig. 3 (not shown), an annular beam is used. The central post electrode is insulated from the cathode by an alumina ceramic sleeve.